MX690 and MX692 Audio Output Level

Question:

How does the output level of the MX690 (measured at the SLX4 receiver's XLR MIC output) compare to the MX692 (measured at the UC4 receiver's XLR MIC output)?

Answer:

The MX690 output is 6 dB less than the MX692. The primary contributor to this level difference is the receiver audio output circuitry. On the SLX4, the XLR output is impedance balanced; pin 2 is driven with the audio signal while pin 3 is not driven. On the UC4, the XLR output is active balanced; pin 2 is driven with the audio signal and pin 3 is also driven with the audio signal - 180 degrees out of polarity compared to pin 2. The voltage swing between pin 2 and pin 3 is 6 dB greater at the UC4 receiver's XLR MIC output.

Here is a summary of our test results:

We set the MX692 at maximum sensitivity (gain pot fully clockwise, attenuator at 0 dB), and the UC4 XLR output at MIC level with the output level control fully clockwise (EQ settings flat). We connected the XLR of the UC4 to channel 1 of an M367 mixer with its input sensitivity set at MIC level. The M367 low-cut switch was activated to reduce effects of low frequency noise in the room.

We positioned an acoustic pink noise generator at a distance of 20 inches in front of the MX692. This corresponds to an acoustic level of about 70 dB SPL at the microphone position, roughly equivalent to the level of conversational speech at that distance.

We set the master output of the M367 at "0" and adjusted the channel one input level to obtain a VU meter reading of 0 VU. We also connected the headphone output of the M367 to a true RMS meter and set the headphone output level to show 0 dB on the 1 volt scale of the meter. This was the reference setup.

With the MX690 at the same distance from the acoustic pink noise generator we connected the XLR output of the SLX4 to the same input of the M367 with all other settings unchanged. We verified that the output attenuator on the SLX4 was set to maximum output (fully clockwise).

Under these conditions, the output level of the MX690/SLX4 combination was about 6 dB less than the output level of the MX692/UC4 combination. For customers using a combination of the MX692 and the MX690, this will be a noticeable difference favoring the MX692. This will require a 6 dB boost on MX690 channels or a corresponding reduction on MX692 channels to equalize levels.

Under these conditions, the audiometer on the UC4 indicated 2 (out of 7) LEDs and the audiometer of the SLX4 also lit up 2 (out of 5) LEDs. It should be noted that bottom three LEDs on the SLX4 roughly track the bottom 4 LEDs of the UC4. However, it is almost impossible to illuminate the upper 2 LEDs on the SLX4 even when the same loud sound illuminates all the LEDs on the UC4. This is likely due to the increased compression ratio of the SLX4 Audio Reference Companding.

Some customers felt the MX692 output level was low compared to a lapel or handheld microphone in the UC series. We expect that the level of the boundary model will be somewhat less than a lapel microphone by virtue of the increased mouth-to-microphone distance. Although this is somewhat offset by the boundary effect (which adds up to 6 dB sensitivity compared to a free-field microphone location) the inverse-square loss will still result in at least a 6 to 8 dB loss for a boundary microphone 36 inches away compared to a lapel microphone 8 inches away.

Customers who questioned the sensitivity of the MX692 were initially concerned because of the low indicator levels on the UC4 audiometer. The resolution is to bring up the levels of the mixer inputs.

There were certain applications that required higher signal levels than could be provided by the MIC level output of the UC4. These customers were advised to set the UC4 XLR output level switch to LINE level. This results in a 30 dB boost compared to the MIC level setting.

There is no LINE level output capability on the SLX4 receiver's XLR output. A customer attempting to add MX690/SLX4 units to an existing MX692/UC4 rig could encounter an even greater discrepancy in the relative output levels of these two systems if the existing UC4 is operating at LINE level out. This difference could be as much as 35 dB.

If a customer using the MX690 and the MX692 in the same setup notices "some" difference between the MX690 and MX692 sensitivities, it should be feasible to raise the gain of the equipment that the MX690 is plugged into by about 6 dB to achieve parity.

However, if a customer indicates a much larger level difference, it is probable that the UC4 output is set to LINE level output. In this case, the UC4 output should first be set to MIC level. Then the input channel of the MX690/SLX4 should be boosted by about 6 dB to achieve parity.

If the output level of the SLX4 needs to be similar to the LINE level output of the UC4, an external preamp must added to provide the additional 30 to 35 dB of gain. One source for such a preamp is Radio Design Labs [www.rdlnet.com].

NOTE: The MX692 and MX690 wireless boundary microphones are designed to operate over a wide dynamic range. This accommodates the voice level of talkers located at different distances from the microphone. It also accommodates the wide range of voice intensities (from calm to heated) experienced in a typical meeting environment. The automatic mixer or teleconferencing device that connect with the MX692 or MX690 must have its gain settings set appropriately to handle the levels provided by the wireless boundary microphones.